1. Field of the Invention
The present invention relates to a method and an apparatus for encoding to generate electronic watermark information, and embedding an electronic watermark into a digital content, a method and an apparatus for reading and decoding the embedded electronic watermark so as to detect tampering, a program for encoding and decoding, and a recording medium for recording this program so as to be read by an information processing apparatus.
2. Description of the Prior Art
Information of a digital content is easily copied on an information processing apparatus such as a computer without degradation. In addition, the information of the digital content is easily tampered by rewriting and the like. Further, the information of the digital content can be easily transmitted on a communication line. As a result, the information of the digital content is copied illegally by a simple process or operation, and then reused without permission. Also, since it is easy to partially tamper the information of the digital content, especially image information pictured by a digital camera, the information is not used as a photographic evidence as a photograph taken by a silver film camera is used.
Methods such as electronic watermark and data hiding are known as methods for preventing the tampering. The electronic watermark is a method for adding information, which is hardly recognized visually when the information is reproduced normally, to a digital content such as digital image information.
This electronic watermark technology is roughly divided into two categories. The first category is technology which directly embeds electronic watermark data into a sampled value of content data. The second category technology which embeds electronic watermark data into a frequency component. With the technology for embedding the electronic watermark data into the sampled value of the content data, though the embedded electronic watermark data tend to be lost when the content data is processed or compressed, the embedding is simple. On the other hand, with the technology for embedding the electronic watermark data into the frequency component, though the electronic watermark is robust against processing and compressing the content data, the processing for embedding and extracting the electronic watermark data is complicated.
A method for uniformly scattering and embedding the electronic watermark data on the entire image, and a method for inserting the electronic watermark only at a remarkable part are known as the technologies for embedding the electronic watermark data into the sampled value of the content data.
A method for adding the electronic watermark data to data in the frequency domain by applying a frequency transform such as fast Fourier transform (FFT), discrete cosine transform (DCT), and wavelet transform to a digital content is known as the technology for embedding the electronic watermark data into the frequency component. When the electronic watermark to be added is such a small quantity that the data do not change original data largely, the data are decoded by inverse frequency transform such that an original image is not visually deteriorated, and simultaneously, the data are transmitted while the electronic watermark information is visually hidden.
Though a computer or a scanner is used to embed the electronic watermark into a digital content, an original data or a content subject to embedding the electronic watermark is necessary. Thus, the embedding the electronic watermark into the digital content using a computer or a scanner is secondary processing which allows tampering before creating the digital content to which the electronic watermark is embedded.
However, when a binary image such as black/white binary image is used as the image information to which the electronic watermark is embedded, a Loss-Less image compression method (such as MH, MR, and MMR which are G3 and G4 standards of the International Telecommunication Union (ITU-T)) which can restore the original image without generating a loss is recommended by the International Telecommunication Union (ITU-T), and this image compression method is generally used. A Lossy image compression technology which may lose detailed information comes to be often used for a multi-valued image such as a color image and a grayscale image while emphasis is placed on data compression for storage rather than avoiding more or less degradation of the image quality. Image compression technology such as JPEG and MPEG using frequency transform based on the discrete cosine transform (DCT) corresponds this technology.
Thus, since the embedded electronic watermark degrades as the image compression degrades the image quality in an application which prevents tampering the multi-valued image, capability for detecting tampering an image decreases, and consequently, there is such a problem that a sufficient function for preventing tampering is not realized.